Extreme Temperatures
Description
Section titled “Description”Extreme Heat
Section titled “Extreme Heat”The Colorado State Hazard Mitigation Plan defines extreme heat as “temperatures over 90 degrees for an extended period of time, or that hover 10 degrees or more above the average high temperature for the region and last for multiple consecutive days.” In a normal year, about 175 Americans succumb to the demands of summer heat. According to the National Weather Service (NWS), among natural hazards, only the cold of winter—not lightning, hurricanes, tornadoes, floods, or earthquakes—takes a greater toll. In the 40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation. In the heat wave of 1980, more than 1,250 people died.
Heat disorders generally have to do with a reduction or collapse of the body’s ability to shed heat by circulatory changes and sweating or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body’s inner core begins to rise, and heat-related illness may develop. Elderly persons, small children, those with chronic illnesses, those on certain medications or drugs, and persons with weight and alcohol problems are particularly susceptible to heat reactions, especially during heat waves in areas where moderate climate usually prevails.
Extreme Cold
Section titled “Extreme Cold”Extreme cold often accompanies a winter storm or is left in its wake. It is most likely to occur in the winter months of December, January, and February. Prolonged exposure to the cold can cause frostbite or hypothermia and can become life-threatening. Infants and the elderly are most susceptible. Pipes may freeze and burst in homes or buildings that are poorly insulated or without heat. Extreme cold can disrupt or impair communications facilities.
Previous Occurrences
Section titled “Previous Occurrences”According to the National Weather Service Forecast Office for Denver/Boulder, there have been 82 streaks with temperatures of 90 degrees or greater since 1895, which accounts for more than 150 days of extremely hot temperatures in the metro area (NWS). During 2008, Denver’s 87-year-old record for the number of consecutive days above 90 degrees Fahrenheit was broken. The new record of 24 consecutive days surpassed the previous record by almost a week. On August 1st, it reached 104 degrees, breaking a record set in 1938 and on August 2nd, it reached 103 degrees, breaking a record set in 1878. In addition, as of August 2008, the area documented 68 days with temperatures above 100°F and 29 days with temperatures below -20°F between February 2008 and 1872 (NWS), as shown in
Figure 4-22 Jefferson County Average Annual Temperature, 1895 – 2020
Section titled “Figure 4-22 Jefferson County Average Annual Temperature, 1895 – 2020”Source: NOAA
By contrast, the Denver Metro area averages 156 days a year with a minimum temperature of 32°F or less. The highest recorded temperature for Jefferson County is 104°F, and the lowest is -41°F. The Southwest Climate and Environmental Information Collaborative (SCENIC) reports data summaries from a station in the City of Lakewood and a station in the Town of Evergreen. contains temperature summaries related to extreme heat for the station.
Table 4-35 Temperature Data from Lakewood and Evergreen Stations
Section titled “Table 4-35 Temperature Data from Lakewood and Evergreen Stations”| Station | Average Annual Maximum Temperature | Average Annual Minimum Temperature | Extreme Maximum Temperature | Extreme Maximum Temperature | Avg Annual Days Max. >90 | Avg Annual Days Max. <32 | Avg Annual Days Min. <32 | Avg Annual Days Min. <0 |
|---|---|---|---|---|---|---|---|---|
| Lakewood1 (054762) | 64 | 37 | 104 6/27/1994 | -26 1/12/1963 | 8.7 | 6.5 | 49.2 | 2 |
| Evergreen2 (052790) | 61 | 27 | 97 6/236/2012 | -38 1/12/1963 | 1.5 | 6.7 | 74.3 | 6 |
Source: SCENIC 1Period of Record: 1962-2020 2Period of Record: 1961-2020
Since temperature variations are a regional hazard, many of the previous occurrences are documented at a regional level as well. For example, between 1996 and 2020 the NCEI database reflects one incident of extreme temperatures for Jefferson County (extreme cold/wind chill in 2011), but documents eight incidents in neighboring Denver County. Therefore, the incidents below impact more than just the planning region.
1983 – A cold spell impacted the entire Metro area with readings dipping to -21°F, marking the coldest recorded temperature in 20 years.
1989 – Periods of extreme cold and high winds combined with snow created a severe storm scenario. Stapleton Airport was closed, and a 46-car pileup occurred on Interstate 25. More details on this storm are captured in Section 4.3.13.
April 11, 1995 – Extreme cold was reported across the region with temperatures recorded at 13°F. Damages to wheat crops in Arapahoe County were estimated at $1 million ($1.4 million in 2008 dollars).
December 16-18, 1996 – Extreme wind chills impacted the entire Front Range and plains regions. Lows in the Denver area were reported at -9°F. A homeless man found in his car, with a body temperature of only 85°F at the time, died a few hours later.
October 24-25, 1997 – A blizzard left snow up to 4′ deep in the foothills and wind gusts were documented at 70 mph. With wind chill, temperatures dropped to between -25°F and -40°F. A State of Emergency was declared, with five recorded deaths and 15 injuries.
December 18-24, 1998 – An arctic air mass settled in over northeastern Colorado dropping overnight temperatures well below zero for 6 consecutive days. Overnight temperatures bottomed out at -19°F on the morning of the 22nd. At least 15 people, mostly homeless, were treated for hypothermia at area hospitals. The bitter cold weather was responsible, either directly or indirectly, for at least 5 fatalities.
Three of the victims died directly from exposure. The cold weather also caused intermittent power outages. Following the cold snap, thawing water pipes cracked and burst in several homes and businesses causing extensive damage. Damage estimates were unavailable.
June and July 2000 – June 29th marked the beginning of a near record hot streak for the Denver area. The maximum high temperature at Denver International Airport equaled or exceeded the 90°F mark for
consecutive days, from June 29th-July 15th; one day short of tying the all-time record. The record of
consecutive days was set in two different years, July 1st-18th, 1874 and July 6th-23rd, 1901.
February 1-4, 2011 – A frigid Arctic air mass settled into the Front Range Urban Corridor to start out the month. At Denver International Airport, overnight low temperatures on the 1st through the 3rd were 13 and 17 below zero and zero respectively. The icy temperatures caused pipes to crack and burst following the freeze. At the Jefferson County Courts administration building, a steady stream of water from a crack on the 5th floor went unnoticed and flooded all floors of the administration wing overnight, damaging much of the office equipment, furniture and carpet. The icy temperatures also forced the closure of several school districts.
Geographic Extent
Section titled “Geographic Extent”The inherent nature of temperature hazards makes them a regional threat, impacting most or all of the planning area simultaneously although the impacts will vary by location. The County being located along the foothills of the Rocky Mountains and encompasses the West Denver Metro area municipalities of Arvada, Golden, Lakewood, Lakeside, Morrison, Mountain View, Westminster and Wheat Ridge. These areas experience similar temperate climate to the remaining Denver Metropolitan Area and are more susceptible to extreme heat events compared to the higher elevations of the County due to the more urbanized areas. The areas of higher elevations like Kittredge, Evergreen, Idledale, and the unincorporated rural mountain areas are more susceptible to extreme variations in general, which can pose a danger to those citizens that may be more vulnerable and certainly so if those extremes temperatures are extended. This is reflected in the previous occurrence record, which consistently discusses the Denver Metro Area, rather than singling out particular counties or communities.
Urbanized areas, in the Denver Metro Area can experience pockets of heightened temperatures where surfaces such as pavement and roofs become hotter than the air temperatures, a phenomenon known as the urban heat island effect. These hot surfaces also retain heat, causing high temperatures to persist even when air temperature drops. Per the EPA, “the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22°F” (US EPA). Colorado’s climate tends to experience large day and night temperature changes. This nighttime cooling will help alleviate heat conditions and is thought to benefit and reduce risk of extreme heat.
The Trust for Public Land, ParkServe online mapping tool allows users to find areas that are impacted by urban heat islands as well as the availability of parks with public access. shows the urban heat island areas within Jefferson County and the level of severity, from mild to severe impact.
Figure 4-23 Urban Heat Island Areas within Jefferson County
Section titled “Figure 4-23 Urban Heat Island Areas within Jefferson County”Source: The Trust for Public Lands, ParkServe https://parkserve.tpl.org/mapping/ The geographic extent rating for extreme temperatures is extensive.
Probability of Future Occurrences
Section titled “Probability of Future Occurrences”Temperature extremes occur on a regular basis, with an annual average of 4.6 days in the mountain areas and 25.7 in the metro area where the maximum temperatures exceed 90°F. The temperatures dip below freezing (32°F) an annual average of 19 days. Severe incidents or prolonged exposures to a temperature extreme are a higher threat to the community than isolated, seasonal occurrences.
There have been 23 incidents of extreme temperatures in Jefferson County since 1961. The methodology for calculating the probability of future occurrences is described in Section This formula evaluates that the probability of a severe temperature extreme occurring in any given year is 39%. This corresponds to a probability of future occurrences rating of likely.
Magnitude and Severity
Section titled “Magnitude and Severity”Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. Since temperature extremes refer to both extreme heat and extreme cold, there is not a single event of record. The event of record for extreme heat in Jefferson County occurred in the summer of 2000. While specific property damages are not available, the event coincided with a severe drought period, which caused extensive damages to crops and personal property, impacted overall water supplies, and caused economic damages due to both conditions. The event of record for
extended periods of severe cold in Jefferson County occurred during December 18-24 in 1998. Damages caused by ruptured water pipes were considered extensive in both the private and public sectors. Power outages increased damages to property and impacted human lives. Hospitals documented a small surge in casualties either directly or indirectly attributed to the cold, and at least 15 injuries were reported. Five deaths were attributed to the cold weather as well, with three of them due directly to exposure.
Nationwide, extreme temperatures remain the leading cause of weather-related deaths.
The National Weather Service Heat Index Program provides a measure of the extent of typical health impacts of exposure to heat, as shown in and During these conditions, the human body has difficulties cooling through the normal method of the evaporation of perspiration, and health risks rise. The chart below illustrates the relationship of temperature and humidity to heat disorders.
Figure 4-24 Heat Index Chart
Section titled “Figure 4-24 Heat Index Chart”Source: National Weather Service
Note that Heat Index (HI) values were devised for shady, light wind conditions. Exposure to full sunshine can increase HI values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.
Table 4-36 Typical Health Impacts of Extreme Heat by Heat Index
Section titled “Table 4-36 Typical Health Impacts of Extreme Heat by Heat Index”| Heat Index | Disorder |
|---|---|
| 80-90° F | Fatigue possible with prolonged exposure and/or physical activity |
| 90-105° F | Sunstroke, heat cramps, and heat exhaustion possible with prolonged exposure and/or physical activity |
| 105-130° F | Heatstroke/sunstroke highly likely with continued exposure |
Source: National Weather Service Heat Index Program,
The NWS has in place a system to initiate alert procedures (advisories or warnings) when the Heat Index is expected to have a significant impact on public safety. The expected severity of the heat determines whether advisories or warnings are issued. A common guideline for the issuance of excessive heat alerts is when the maximum daytime high is expected to equal or exceed 105°F and a nighttime minimum high of 80°F or above is expected for two or more consecutive days.
In 2001, the NWS implemented an updated Wind Chill Temperature index (see ). This index was developed to describe the relative discomfort/danger resulting from the combination of wind and temperature. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature.
Figure 4-25 National Weather Service Wind Chill Chart
Section titled “Figure 4-25 National Weather Service Wind Chill Chart”Source: National Weather Service
The National Weather Service Denver/Boulder Forecast Office issues warnings and advisories for cold temperatures. The following is a breakdown on the various NWS defined watches, warnings and advisories that could be issued:
Wind Chill Watch is issued when wind chill warning criteria are possible in the next 12 to 35 hours.
Wind Chill Warning is issued for wind chills of at least -25°F on the plains and -35°F in the mountains and foothills.
Wind Chill advisory is issues on the plains when wind and temperature combine to produce wind chill values of -18°F to -25°F and -25°F for the mountains and foothills.
Freeze Watch is issued when freeze conditions are possible in the next 12 to 36 hours.
Freeze Warning is issued during the growing season when widespread temperatures are expected to drop to below 32°F.
A frost advisory is issued during the growing season when temperatures are expected to drop to between 32°F and 35°F on clear calm nights.
The Jefferson County Emergency Preparedness Guide addresses both of these temperature extremes, and notes that people living in urban areas may experience a greater risk from the effects of a prolonged heat wave than those living in rural areas, due to the impacts of heat on the atmosphere, air quality and temperature. In some cases, extreme heat incidents may lead to emergency water shortages, which are shorter in duration than a drought, but exhibit similar impacts and secondary hazardous situations.
Based on these factors, the magnitude and severity rating for temperature extremes is considered
limited.
Section titled “limited.”Climate Change Considerations
Climate change is projected to increase the uncertainty of weather patterns and produce more extreme climate induced events. Scientists have suggested that warming in the Artic has been linked changes in the jet stream which may lead to increased polar vortex events in Colorado. The polar vortex is well documented and is described as large areas of low pressure and cold air surrounding the North and South poles. Increased temperatures in the polar regions has weakened and destabilized the jet stream leading to polar air to dip into lower latitudes, bringing it farther south than typical (UC Davis).
Research cited in the Fourth National Climate Assessment indicates that average temperatures have already increased across the Southwest and will likely continue to rise. shows the difference between the 1986-2016 average temperature and the 1901-1960 average temperature. This trend toward higher temperatures is expected to continue and would cause more frequent and severe droughts in the Southwest as well as drier future conditions and an increased risk of megadroughts—dry periods lasting 10 years or more). Additionally, current models project decreases in snowpack, less snow and more rain, shorter snowfall seasons, and earlier runoff, all of which may increase the probability of future water shortages (Gonzalez et al., 2018).
Figure 4-26 Change in Average Temperature Across the Southwest, 1901-1960 to 1986-2016
Section titled “Figure 4-26 Change in Average Temperature Across the Southwest, 1901-1960 to 1986-2016”Source: Fourth National Climate Assessment
Extreme heat is also expected to increase in frequency. shows projected increases in extreme heat as an increase in the number of days per year when the temperature exceeds 90°F by the period 2036-2065 compared to the period 1976-2005. Under the higher emissions scenario (RCP8.5), the number of days of extreme heat would increase in Jefferson County by 30 to 50 days based on the figure below.
Figure 4-27 Projected Increases in Extreme Heat
Section titled “Figure 4-27 Projected Increases in Extreme Heat”Source: Fourth National Climate Assessment *Based on higher emission scenario RCP8.5
Vulnerability Assessment
Section titled “Vulnerability Assessment”General Property
Section titled “General Property”Recent research indicates that the impact of extreme temperatures, particularly on populations, has been historically under-represented. The risks of extreme temperatures are often profiled as part of larger hazards, such as severe winter storms or drought. However, as temperature variances may occur outside of larger hazards or outside of the expected seasons but still incur large costs, it is important to examine them as stand-alone hazards. Extreme heat may overload demands for electricity to run air conditioners in homes and businesses during prolonged periods of exposure and presents health concerns to individuals outside in the temperatures. Extreme heat may also be a secondary effect of droughts or may cause temporary drought-like conditions. For example, several weeks of extreme heat increases evapotranspiration and reduces moisture content in vegetation, leading to higher wildfire vulnerability for that time period even if the rest of the season is relatively moist.
Extreme cold impacts structures when pipes or water mains freeze and burst, causing damage. Cold can also, in the most extreme of circumstances, make materials more fragile and breakable, although the Front Range rarely gets this cold. Extreme cold may also lead to higher electricity and natural gas demands to maintain appropriate indoor heating levels combined with damages caused to the delivery infrastructure such as frozen lines and pipes. Cold may impact transportation as well. Exposed populations may be at risk while waiting for public transportation, particularly when combined with wind- chill, and some vehicles may not start which impacts the commute of the workforce and, in worst case scenarios, the movement of emergency services personnel.
People
Section titled “People”The impacts of cold and extreme heat on health are also a consideration. Traditionally, the very young and very old are considered at higher risk to the effects of extreme temperatures, but any populations outdoors in the weather are exposed, including otherwise young and healthy adults and homeless populations. Arguably, the young-and-otherwise-healthy demographic may be more exposed and experience a higher vulnerability because of the increased likelihood that they will be out in the extreme temperature deviation, whether due to commuting for work or school, conducting property maintenance such as snow removal or lawn care, or for recreational reasons.
Critical Facilities and Infrastructure
Section titled “Critical Facilities and Infrastructure”Prolonged heat exposure can have significant impacts on infrastructure. Prolonged high heat exposure increases the potential of pavement deterioration, as well as railroad warping or buckling. High heat also puts a strain on energy systems and consumption, as air conditioners are run at a higher rate and for longer. Extreme heat can also reduce transmission capacity over electric systems.
Secondary impacts of extreme cold can affect the supporting mechanisms or systems of a community’s infrastructure. For example, when extreme cold is coupled with high winds or ice storms, power lines may be downed, resulting in an interruption in the transmission of that power shutting down electric furnaces, which may lead to frozen pipes in homes and businesses.
The impact of severe temperature deviation on power delivery is a significant factor when assessing current development exposure. Xcel Energy, the utility provider for Jefferson County, estimates that service outages due to extreme temperatures cost the utility an average of $50,000 to fix for every 20,000 people affected. This includes repair and replacements costs, equipment usage and crew overtime.
Economy
Section titled “Economy”Extreme temperatures can lead to potential loss of facilities or infrastructure function or accessibility and uninsured damages. Impact to transportation sector and movement of goods. Historic events in Colorado have impacted community business districts where a majority of businesses are lost (CO SHMP 2018).
Historical, Cultural, and Natural Resources
Section titled “Historical, Cultural, and Natural Resources”Jefferson County has hundreds of square miles of parks and open space which provide habitat for various species that are valuable to residents and visitors to the County, and which are vulnerable to extreme temperatures. (Jefferson County 2018). Extreme temperatures can have significant impacts to these natural ecosystems. Increasing temperatures may cause species to shift habitats in elevation and latitude and extended periods of extreme heat can stress both flora and fauna species. According to Colorado Parks and Wildlife, warmer temperatures can also lead to earlier snowmelt affecting insect and wildlife life cycles as well as seed production and germination.
Future Development
Section titled “Future Development”Since structures are not usually directly impacted by severe temperature fluctuations, continued development is less impacted by this hazard than others in the plan. However, new development can add stress to the electric grid, potentially increasing the possibility of brownouts or blackouts.
Pre-emptive cautions such as construction of green buildings that require less energy to heat and cool, use of good insulation on pipes and electric wirings, and smart construction of walkways, parking structures, and pedestrian zones that minimize exposures to severe temperatures may help increase the overall durability of the buildings and the community to the variations. Continued development also implies continued population growth, which raises the number of individuals potentially exposed to variations. Public education efforts should continue to help the population understand the risks and vulnerabilities of outdoor activities, property maintenance, and regular exposures during periods of extreme heat and cold.
Overall Hazard Significance
Section titled “Overall Hazard Significance”Extreme temperatures in Jefferson County have a particular impact on the planning area. The risk to the population is the greatest, with exposure posing a significant threat to life and safety of residents. In addition, potential damages to property as an indirect impact of the temperature, particularly during cold weather, are costly. Temperature extremes often accompany other, more obvious hazards such as droughts and blizzards or other winter storms and may have undocumented impacts in the community as well. The geographic extent of the hazard is considered extensive. The probability of future occurrences is considered likely and the magnitude/severity for the events of record is limited. The HMPC considers the hazard to have an overall impact rating of low on the County. Collectively, the data indicates that the overall impact rating for extreme temperatures is low.